API Testing Tutorial

What is API?

API (Full form Application Programming Interface) enables communication and data exchange between two separate software systems.

A software system implementing an API contains functions/sub-routines which can be executed by another software system.

It comprises of a set of functions that can be accessed and executed by another software system. Thus, it serves as an interface between different software systems and establishes their interaction and data exchange.

Real life Example of API:
1> Ordering food on restaurant
2> Login to 3rd Party App using Google or Facebook Login
3> Slack Notification

What is Web Service?

A Web Service is a service accessed via Web. Web Service is a way to publish your application over web and enable other applications to access functions defined by your web service.

Difference Between API and Web Services

Benefits of API

Benefits of API : API testing is the only way to provide truly secure, reliable and scalable connections between platforms. Testing provides these benefits.

1. Access to application without user interface
2. Protection from malicious code and breakage
3. Time Efficiency vs. functional and validation testing
4. Cost Effective / Reduces Testing Cost
5. Technology Independent

What is API Testing?

API testing is a software testing type that validates Application Programming Interfaces (APIs).

The purpose of API Testing is to check the functionality, reliability, performance, and security of the programming interfaces. In API Testing, instead of using standard user inputs(keyboard) and outputs, you use software to send calls to the API, get output, and note down the system's response.

API tests are very different from GUI Tests and won't concentrate on the look and feel of an application.

It mainly concentrates on the business logic layer of the software architecture.

API Testing vs Unit Testing

Types of Output of API

Any type of data
If we are return any value to the API like submitting input values and getting output value

Status (say Pass or Fail)
If any thing goes pass or fail like value or label or message display or match value assert. Lock, Unlock, Delete status as well.

Call another API function
First API function can be used for deleting a specified record in the table and this function, in turn, calls another function to REFRESH the database.

Test Cases for API Testing

Return value based on input condition: It is relatively easy to test, as input can be defined and results can be authenticated

Does not return anything: When there is no return value, a behavior of API on the system to be checked

Trigger some other API/event/interrupt: If an output of an API triggers some event or interrupt, then those events and interrupt listeners should be tracked

Update data structure: Updating data structure will have some outcome or effect on the system, and that should be authenticated

Modify certain resources: If API call modifies some resources then it should be validated by accessing respective resources

Type of APIs

REST : (REpresentational State Transfer)

REST is an architectural style that uses simple HTTP calls for inter-machine communication. REST does not contain an additional messaging layer and focuses on design rules for creating stateless services.

A client can access the resource using the unique URI and a representation of the resource is returned. With each new resource representation, the client is said to transfer state.

While accessing RESTful resources with HTTP protocol, the URL of the resource serves as the resource identifier and GET, PUT, DELETE, POST and HEAD are the standard HTTP operations to be performed on that resource

SOAP : (Simple Object Access Protocol)

SOAP relies heavily on XML, and together with schema, defines a very strongly typed messaging framework. Every operation the service provides is explicitly defined, along with the XML structure of the request and response for that operation.

Each input parameter is similarly defined and bound to a type: for example, an integer, a string, or some other complex object. All of this is codified in the WSDL – Web Service Description (or Definition, in later versions) Language. The WSDL is often explained as a contract between the provider and the consumer of the service.

Difference Between SOAP & REST

Best Practices of API Testing

Test cases should be grouped by test category
On top of each test, you should include the declarations of the APIs being called.
Parameters selection should be explicitly mentioned in the test case itself
Prioritize API function calls so that it will be easy for testers to test
Each test case should be as self-contained and independent from dependencies as possible
Avoid "test chaining" in your development
Special care must be taken while handling one-time call functions like - Delete, Close Window, etc...
Call sequencing should be performed and well planned
To ensure complete test coverage, create test cases for all possible input combinations of the API.